1. Field of the Invention
The invention relates generally to an optical network system, including for example a broadband passive optical network (B-PON). In particular, the invention describes an optical network system that allows alternative access techniques (such as for example an Ethernet-type CSMA/CD MAC) to be implemented optically on the network.
2. Description of Related Technology
While optical networking as an industry has experienced significant growth over the past few years, this growth is mainly focused on long haul or backbone optical networks. Although new technologies that allow cost-effective, scalable, reliable, high-bandwidth services are emerging in the metro and regional market, little has changed in the access arena. The ever-increasing demand for bandwidth has accelerated the lag of subscriber access network capacity. In other words, the bandwidth bottleneck has evolved outward from the core to the subscriber access network, or the so-called “first mile.” In order to be able to provide the new services that customers demand, service providers need to find ways to offer higher data rates at reasonable costs.
Several technologies exist today that are being used to increase the capacity of subscriber access networks. For example, hybrid fiber-coaxial (HFC) and digital subscriber line (DSL) networks are being deployed by many service providers. The HFC architecture utilizes optical fiber to transport data from the head-end to a curbside optical node in the neighborhood. The final distribution to the subscriber homes is performed by coaxial cable using a bus architecture. While this is a relatively low cost evolution of widely deployed CATV fiber-node architecture, it suffers from very low throughput during peak hours due to the large number of subscribers who share the bandwidth provided by the optical node. DSL uses the same twisted pair as telephone lines and requires a DSL modem at the customer premises and a digital subscriber line access multiplexer (DSLAM) in the central office. While the data rate (128 Kb/s -1.5 Mb/s) offered by DSL is significantly higher than that of an analog modem, it can hardly be considered as broadband as it cannot support full-service voice, data, and video. In addition, it suffers the distance limitation as one central office can only cover distances less than approximately 5.5 km using present technologies.
In the past several years, two technologies have begun to emerge as alternatives for optical access: (i) point-to-point Gigabit Ethernet, and (ii) passive optical network (PON). Point-to-point is the topology Ethernet has used successfully for a decade. It is a logical way to deploy optical fiber in the subscriber access network. However, with dedicated fiber running from the central office to each subscriber, this architecture has very low fiber utilization (N fibers and 2N transceivers are needed for N subscribers). While the technology has its merits, such as very easy bandwidth provisioning and its excellence for native LAN extension service, in many cases, it is viewed by many as a less attractive solution for small- to medium-sized businesses and residences. An alternative to the point-to-point Ethernet is the curb-switched Ethernet where a remote switch is deployed near the neighborhood. While this reduces the fiber consumption from N fibers to 1 fiber, the number of transceivers increases by 2 to 2N+2. In addition, a curb-switched Ethernet requires active components and therefore electrical power in the field, an unfavorable situation due to, inter alfa, high cost.
Passive optical networks (PONs) are low cost Fiber-to-the-Building/Curb/Home (FTTb, FTTc, FTTh, collectively referred to as FTTx) solutions. A PON is a point-to-multipoint optical network that allows service providers to minimize the need for fiber in the outside portion of the network to interconnect buildings or homes. The basic principle of PON is to share the central optical line terminal (OLT) and the feeder-fiber by as many optical network units (ONUs) as is practical. This resource sharing allows a significant reduction of network capital expense allocated to each subscriber and therefore enables broadband fiber access in areas where achieving profitability has been a formidable task for traditional point-to-point or ring-based architectures.
A typical prior art PON model 100 is shown in FIG. 1, and consists of four elements: an optical line terminal (OLT) 102, a plurality of optical network units (ONUs) 104, an optical distribution network, also know as the outside plant (OSP) 106, and an element management system (EMS) 108. The OLT 102 typically resides in the central office (CO), serving as the interface between the PON system and the service provider's core networks 110. The ONUs 104 are located at either the curb or the end-user location, serving as the interface between the PON system 100 and broadband service customers 112. The optical distribution network 106 includes single-mode fiber optic cable, passive optical splitters/couplers, connectors and splices. The element management system (EMS) 108 manages a plurality of PONs and there respective nodes. It offers network management functions in areas including fault detection/isolation, configuration, accounting, performance, and security. The OLT and ONUs are now described in more detail.
The OLT 102 in a typical asynchronous transfer mode (ATM) PON system consists of three parts: (i) the service port function; (ii) an ODN interface; and (iii) a MUX for VP grooming.
The service port function serves as an interface to service nodes. The service port function inserts ATM cells into the upstream SONET/SDH payload and extracts ATM cells from the SONET/SDH payload.
The optical distribution network interface performs optoelectronic conversion. It inserts ATM cells into the downstream PON payload and extracts ATM cells from the upstream PON payload.
Lastly, the MUX provides VP connections between the service port function and the ODN interface.
An ONU in an ATM PON system consists of an ODN interface, user port, transmission, customers and services, mux/demux functions, and powering. The ODN interface performs the optoelectronic conversion. The ODN interface extracts ATM cells from the downstream payload and inserts ATM cells into the upstream PON payload. The MUX multiplexes service interfaces to the ODN interface(s). The user port interfaces over UNI to a terminal, and inserts ATM cells into the upstream payload and extracts ATM cells from the downstream payload. ONU powering is typically implementation dependent.
The wavelength window of PON is typically in the 1.5 μm region for downstream and 1.3 μm region for upstream to support a single fiber system. Downstream traffic is transmitted from the central office towards the optical star coupler where light signal is passively split and distributed by a plurality of optical fibers to a plurality of optical network units (ONUs). The ONUs provide data, voice, and video services to the end subscriber(s) electronically. In the upstream direction, the respective signals from the ONUs are passively combined by the optical star coupler. The combined optical signal is then distributed to the central office through a single optical fiber. Some proposed PON schemes utilize wavelengths other than 1.5 μm/1.3 μm or multiplex additional wavelengths to support an analog/digital video overlay on the same fiber. Others use a second PON (video PON) to provide video services. The video PON is typically provided on a parallel fiber that has the same physical layout as the first PON.
To date, PON-based optical access networks have primarily been designed to use asynchronous transfer mode (ATM) as its layer 2 protocol (ITU Std. G.983), and thus the term “APON.” ATM was chosen because it was considered to be suitable for multiple protocols. In this scheme, both downstream and upstream data are formatted to fit into the fixed time slot cell structure (e.g., 53 bytes) of ATM. In the downstream direction, data is broadcast at 1550 nm using Time Division Multiplexing (TDM) protocol for point-to-multipoint transmission. In the upstream direction, 1310 nm is used over which a Time Division Multiple Access (TDMA) protocol is applied providing the multipoint-to-point shared medium access.
More recently, however, Ethernet (IEEE Std. 802.3) has emerged as a universally accepted standard, with several hundred millions of Ethernet ports deployed worldwide. This large-scale deployment has steadily driven the prices of standard Ethernet devices down. As of this writing, the deployment of Gigabit Ethernet is increasing and 10 Gigabit Ethernet products are becoming more and more available. In addition to its economic advantages, Ethernet is in many ways a logical candidate for an IP data optimized access network. An Ethernet PON (EPON) is a PON in which both downstream and upstream data are encapsulated in Ethernet frames. For the most part, an EPON is very similar to an APON (see FIG. 1) in that the network topology is architecturally similar and adheres to many G.983 recommendations. In the downstream direction, Ethernet frames are broadcast at 1550 nm through the 1:N passive star coupler and reach each ONU. Since broadcasting is one of the key characteristics of Ethernet, it makes logical sense to use Ethernet frames in the PON architecture. In the upstream direction, traffic is managed utilizing time-division multiple access (TDMA). In this scheme, transmission time slots are allocated to all of the ONUs. The time slots are synchronized so that upstream data from the ONUs do not collide with each other once the data are coupled onto the single common fiber. For the purposes of the present discussion, both APONs and EPONs are referred to herein as PONs.
Although the art of transmitting data from central office to user and from user to central office is well developed based on either APON or EPON technology, certain problems still exist. In particular, in the upstream direction, due to the directional properties of the optical star coupler, data from any ONU 104 will only reach the OLT 102, and not other ONUs. Thus, simultaneously transmitted data from different ONUs 104 use a time-sharing mechanism (i.e. TDMA) to avoid collision. Some other methods for avoiding upstream collisions include installing an Ethernet hub at the star coupler (and thereby effectively defeating the purpose of being passive), and using wave-division multiplexing (WDM) to separate one ONU from another. This latter approach is quite cost prohibitive. Due to the lack of their popularity, these latter two methods (i.e., use of the Ethernet hub and WDM) are not discussed further herein.
While TDMA does provide the scheduling capability, it also imposes more complexity on hardware and protocol software. For example, both OLT and ONU must be able to manage and process the transmitted and received data in terms of timeslots and frames as well as perform frame synchronization. Clearly, these requirements cannot be easily satisfied by conventional Ethernet or non-Ethernet devices. In addition, in order to avoid upstream frame collision in an APON (and presumably EPON as well), the OLT 102 must perform an operation known as “ranging” in which the OLT measures the distance to each ONU 104, and then tells the ONU 104 to insert the appropriate delay so that all equivalent OLT-ONT distances are a predetermined value, e.g., 20 km. The ranging procedure complicates the protocol software significantly.
Another common problem associated with existing passive optical networks is the so-called “near-far” problem. This problem is caused by unequal distances between the central office and various ONUs. The longer the distance, the lower the power level received at the OLT 102. A number of approaches have been considered to overcome this problem. For example, a burst mode OLT receiver that is able to quickly adjust its zero-one threshold at the beginning of each received time slot can be used to detect the incoming bit-stream correctly. Alternatively, a special OLT-ONU signaling protocol can be developed that allows the ONUs to adjust their respective transmitter power based on OLT feedback such that power levels received by the OLT from all the ONUs are the same. While these methods do solve the near-far problem, they require highly sophisticated hardware and/or software, thereby increasing complexity as well as cost of implementation.
It is noted that even in the case of EPON, as long as TDMA is used to resolve upstream frame collision, significant amount of hardware and software components are required at both the OLT 102 and ONU 104. These components are not the types used on enterprise Ethernet networks and therefore may not benefit from the volume advantage of standard Ethernet devices.
Additionally, it will be recognized that the operation of Ethernet is largely based on the contention-based media access protocol CSMA/CD along with the back-off algorithm (10 Gigabit Ethernet not included). This MAC layer protocol has many desirable characteristics such as simplicity and being a well understood and proven technology. Therefore, it is highly desirable to operate the PON using Ethernet-like MAC protocols.
Based the foregoing, it is clear that a need exists for an improved optical networking architecture and methods that take advantage of the popularity of widely accepted communications standards and protocols (such as Ethernet) and their high deployment scale. Ideally, such architecture and methods would employ a simple medium access protocol (such as the aforementioned CSMA/CD), and allow the use of low-cost network interface devices based on these widely accepted standards and protocols.